This invention relates to an antenna construction for a radio transmitter or receiver to be carried on the wrist, and more particularly relates to an expansion band antenna for a wristwatch radio device.
Several proposals are known for antennas for small portable radios in which the antenna is incorporated into a belt or strap which also supports the radio on the person of the user. Examples of these are shown in U.S. Pat. No. 2,470,687 issued to Cafrella et al. on May 17, 1949, U.S. Pat. No. 3,523,296 issued to Vliegenthardt on Aug. 4, 1970, U.S. Pat. No. 2,255,897 to Rebori et al. on Sept. 16, 1941, and U.S. Pat. No. 4,340,972 issued to Heist on July 20, 1982. The Heist and Vliegenthardt patents depict antennas designed to function as conventional dipoles. The Cafrella et al. patent shows a loop antenna stitched between two plies of a supporting belt, and the Rebori patent depicts a loop antenna with a parallel connected tuning capacitor and coupled to a crystal "detector."
Proposals are also known for combining a radio transmitter or receiver with a timepiece and arranging the antenna for the transmitter or receiver inside two separate halves of a wristband, the conductors in each half being connected to the radio device inside the timepiece case. An example is shown in U.S. Pat. No. 3,032,651 to Gisiger-Stahli et al. on May 1, 1962 having serpentine conductors folded back and forth longitudinally along the halves. Another proposal appears in published European patent application No. 0 100 639 A2 published Feb. 15, 1984 in the name of Sinclair Research Limited. A continuous watchband is shown with transversely oriented loops strung on a pair of conductors running longitudinally and embedded in the watchband, the separate loops being wound on ferrite cores.
A proposal for a wristwatch receiver antenna is disclosed in PCT application, Internation Publication No. WO 86/03645 published June 19, 1986 in the name of AT&E Corporation, in which the watchband comprises two sections of a strip conductor within a strap fastened by a conductive clasp or buckle. This construction requires special grommets on one side to make connection with the tongue of the buckle on the other side or use of a conductive clasp. Such proposals introduce the possibility of electrical discontinuities the midpoint of the antenna. An alternate proposal in the aforesaid application was to zig-zag a conductor through successive links of a metal expansion band, a tedious and expensive procedure.
Normally an antenna is designed in an effort to attain ideal physical dimensions corresponding to half of a wave length, or dipole. However, a wristwatch antenna is unable to achieve an effective length corresponding to an ideal dipole and is thus obliged to transfer energy within the constraints of the physical size of the wrist instrument. If the antenna is small, the greatest power transfer to a circuit requires impedance matching with a resonant tuned antenna circuit. The theory of small antennas is set forth in Small Antennas by Harold A. Wheeler published in IEEE Transactions and Antennas and Propogation, volume AP-23, No. 4, July 1975 and also in an article entitled "Loop Antennas" by Glenn S. Smith, pages 5-2 through 5-9 appearing in Antenna Engineering Handbook, Second Edition, published by McGraw Hill, 1984. As shown in these articles, when the greatest antenna dimension is less than one-quarter wave length, and typically much smaller than that, a small loop antenna is analyzed as a radiating inductor, with impedance matching required to achieve the best power transfer. The greatest physical dimension of a wrist instrument is the diameter of the strap or band, which practically can be no greater than around 7.5 cm. For example, for transmission or reception at 40 MHz (wave length of approximately 7.5 meters), a wristwatch antenna is no larger than approximately 0.01 times the wave length at this frequency.
One of the requirements for a wrist instrument is to be able to get the instrument off and on the wrist. This either requires a buckle or clasp, or an expansion band. An expansion band allows a continuous conductor without electrical discontinuities which might degrade its performance as an antenna. However, an expansion band must also be flexible and able to expand and contract without breaking or affecting the performance of the antenna wire associated with it.
Accordingly, one object of the present invention is to provide an improved expansion band antenna for a wristwatch transmitter/receiver.
Another object of the invention is to provide an improved antenna for a wrist instrument having stretchable and flexible qualities.
Another object of the invention is to provide an improved method of making an expansion band antenna.